Trinucleotide repeat expansion disorders were first characterized in the early 1990s (see Di Prospero and Fischbeck, (2005) Nature Reviews Genetics vol 6:756-765). These disorders involve the localized expansion of unstable repeats of sets of three nucleotides and can result in loss of function of the gene in which the repeat resides, a gain of toxic function, or both. Trinucleotide repeats can be located in any part of the gene, including non-coding and coding gene regions. Repeats located within the coding regions typically involve either a repeated glutamine encoding triplet (CAG) or an alanine encoding triplet (CGA). Expanded repeat regions within non-coding sequences can lead to aberrant expression of the gene while expanded repeats within coding regions (also known as codon reiteration disorders) may cause mis-folding and protein aggregation. The exact cause of the pathophysiology associated with the aberrant proteins is often not known. Typically, in the wild-type genes that are subject to trinucleotide expansion, these regions contain a variable number of repeat sequences in the normal population, but in the afflicted populations, the number of repeats can increase from, in some cases a simple doubling in the number of repeats, to a log order increase in the number of repeats. For example, in the FMR1 gene, which is subject to CGG expansion in Fragile X patients, the wild-type population displays from 2-50 repeats, while those patients afflicted with Fragile X syndrome can have 200-2000 CGG repeats (Nadel et al., (1995) Journal Biological Chemistry 270 (48):28970-28977).
To date, 20 different disorders have been linked to expanded trinucleotide repeats (see Di Prospero and Fischbeck ibid). The phenomenon was first described in spinal and bulbar muscular atrophy (SBMA) wherein a CAG repeat is expanded in a coding region of the androgen receptor. The repeat in the wild-type gene normally comprises 13 to 30 CAGs while SBMA patients can have as many as 40 or more. Other disorders characterized by expanded trinucleotide repeats include Freidreich ataxia (repeats are in the non-coding region of the Fratazin gene), Fragile X Syndromes A and E (repeats are in the non-coding regions of the FMR1 and FMR2 gene, respectively), and Huntington Disease, where repeats are inserted within the N terminal coding region of the large cytosolic protein Huntingtin (Htt). Each polyglutamine expansion disorder displays characteristic pathology, with neuronal loss evident in specific regions of the brain. Polyglutamine expansions in the P/Q Ca2+ channel, in the TATA box binding protein, and in atrophin-1 give rise to spinocerebellar ataxia (SCA)-6, SCA-17, and dentatorubralpallidoluysian atrophy (DRPLA) respectively. Apart from their polyglutamine repeats, the proteins involved in these disorders are unrelated, although they are all widely expressed in both the central nervous system and peripheral tissues.
Huntington's Disease (HD), also known as Huntington's Chorea, is a progressive disorder of motor, cognitive and psychiatric disturbances. The mean age of onset for this disease is age 35-44 years, although in about 10% of cases, onset occurs prior to age 21, and the average lifespan post-diagnosis of the disease is 15-18 years. Prevalence is about 3 to 7 among 100,000 people of western European descent. Normal Htt alleles contain 15-20 CAG repeats, while alleles containing 35 or more repeats can be considered potentially HD causing alleles and confer risk for developing the disease. Alleles containing 36-39 repeats are considered incompletely penetrant, and those individuals harboring those alleles may or may not develop the disease (or may develop symptoms later in life) while alleles containing 40 repeats or more are considered completely penetrant and no asymptomatic persons containing HD alleles with this many repeats have been reported. Those individuals with juvenile onset HD (<21 years of age) are often found to have 60 or more CAG repeats. In addition to an increase in CAG repeats, it has also been shown that HD can involve +1 and +2 frameshifts within the repeat sequences such that the region will encode a poly-serine polypeptide (encoded by AGC repeats in the case of a +1 frameshift) track rather than poly-glutamine (Davies and Rubinsztein (2006) Journal of Medical Genetics 43:893-896).
Huntington's Disease is a genetic disease where the HD allele is usually inherited from one parent as a dominant trait. Any child born of a HD patient has a 50% chance of developing the disease if the other parent was not afflicted with the disorder. In some cases, a parent may have an intermediate HD allele and be asymptomatic while, due to repeat expansion, the child manifests the disease. In addition, the HD allele can also display a phenomenon known as anticipation wherein increasing severity or decreasing age of onset is observed over several generations due to the unstable nature of the repeat region during spermatogenesis.
In HD, trinucleotide expansion leads to neuronal loss in the medium spiny gamma-aminobutyric acid (GABA) projection neurons in the striatum, with neuronal loss also occurring in the neocortex. Medium spiny neurons that contain enkephalin and that project to the external globus pallidum are more involved than neurons that contain substance P and project to the internal globus pallidum. Other brain areas greatly affected in people with Huntington's disease include the substantia nigra, cortical layers 3, 5, and 6, the CA1 region of the hippocampus, the angular gyms in the parietal lobe, Purkinje cells of the cerebellum, lateral tuberal nuclei of the hypothalamus, and the centromedialparafascicular complex of the thalamus (Walker (2007) Lancet 369:218-228). The role of the normal Htt protein is poorly understood, but may be involved in neurogenesis, apoptotic cell death, and vesicle trafficking. In addition, there is evidence that wild-type Htt stimulates the production of brain-derived neurotrophic factor (BDNF), a pro-survival factor for the striatal neurons. It has been shown that progression of HD correlates with a decrease in BDNF expression in mouse models of HD (Zuccato et al., (2005) Pharmacological Research 52(2):133-139), and that delivery of either BDNF or glial cell line-derived neurotrophic factor (GDNF) via adeno-associated viral (AAV) vector-mediated gene delivery may protect straital neurons in murine models of HD (Kells et al., (2004) Molecular Therapy 9(5):682-688).
Treatment options for HD are currently very limited. Some potential methodologies designed to prevent the toxicities associated with protein aggregation that occurs through the extended poly-glutamine tract such as overexpression of chaperonins or induction of the heat shock response with the compound geldanamycin have shown a reduction in these toxicities in in vitro models. Other treatments target the role of apoptosis in the clinical manifestations of the disease. For example, slowing of disease symptoms has been shown via blockage of caspase activity in animal models in the offspring of a pairing of mice where one parent contained a HD allele and the other parent had a dominant negative allele for caspase 1. Additionally, cleavage of HD Htt by caspase may play a role in the pathogenicity of the disease. Transgenic mice carrying caspase-6 resistant mutant Htt were found to maintain normal neuronal function and did not develop striatal neurodegeneration as compared to mice carrying a non-caspase resistant mutant Htt allele. (see Graham et al., (2006) Cell 125:1179-1191). Molecules which target members of the apoptotic pathway have also been shown to have a slowing affect on symptomology. For example, the compounds zVAD-fmk and minocycline, both of which inhibit caspase activity, have been shown to slow disease manifestation in mice. The drug remacemide has also been used in small HD human trials because the compound was thought to prevent the binding of the mutant Htt to the NDMA receptor to prevent the exertion of toxic affects on the nerve cell. However, no statistically significant improvements were observed in neuron function in these trials. In addition, the Huntington Study Group conducted a randomized, double-blind study using Co-enzyme Q. Although here was a trend towards slower disease progression among patients that were treated with coenzyme Q10, there was no significant change in the rate of decline of total functional capacity. (Di Prospero and Fischbeck, ibid).
Thus, there remains a need for compositions and methods for the treatment of trinucleotide repeat disorders.